The present invention relates to the preparation of microscopic drug delivery systems (MDDS) utilizing drug-encapsulating bioadhesive liposomes.
Currently, the topical and local administration of a drug can be in its free form, dissolved or dispersed in a suitable diluent, or in a vehicle such as a cream, gel or ointment. By definition, "topical" administration includes non-invasive drug administration, while "local" includes invasive, i.e., through a localized injection or infusion. Examples of therapeutic or designated targets for topical or local drug administration include burns; wounds; bone injuries; ocular, skin, intranasal and buccal infections; ocular chronic situations such as glaucoma; intraperitoneal infections, tumors and metastasis; and topically and locally accessed tumors. Several difficulties exist with either the topical or local administration of a drug in its free form. For example, short retention of the drug at the designated site of administration reduces the efficacy of the treatment and requires frequent dosing. Exposure of the free form drug to the biological environment in the topical or local region can result in drug degradation, transformation into inactive entities and nondiscriminating and uncontrollable distribution of the drug. Such degradation and uncontrollable distribution of the drug can result in toxicity issues, undesirable side effects and loss of efficacy.
Microscopic drug delivery systems (MDDS) have been developed for improved drug administration relative to administration of drugs in their free form. Drug-loaded MDDS can perform as sustained or controlled release drug depots. By providing a mutual protection of the drug and the biological environment, MDDS reduces drug degradation or inactivation. As a system for controlled release of a drug, MDDS improves drug efficacy and allows reduction in the frequency of dosing. Since the pharmacokinetics of free drug release from depots of MDDS are different than from directly-administered drug, MDDS provides an additional measure to reduce toxicity and undesirable side effects.
MDDS is divided into two basic classes: particulate systems, such as cells, microspheres, viral envelopes and liposomes; or nonparticulate systems which are macromolecules such as proteins or synthetic polymers. Liposomes have been studied as drug carriers and offer a range of advantages relative to other MDDS systems. Composed of naturally-occurring materials which are biocompatible and biodegradable, liposomes are used to encapsulate biologically active materials for a variety of purposes. Having a variety of layers, sizes, surface charges and compositions, numerous procedures for liposomal preparation and for drug encapsulation within them have been developed, some of which have been scaled up to industrial levels.
Liposomes can be designed to act as sustained release drug depots and, in certain applications, aid drug access across cell membranes. Their ability to protect encapsulated drugs and various other characteristics make liposomes a popular choice in developing MDDS, with respect to the previous practices of free drug administration.
Despite the advantages offered, utilization of drug-encapsulating liposomes does pose some difficulties. For example, liposomes as MDDS have limited targeting abilities, limited retention and stability in circulation, potential toxicity upon chronic administration and inability to extravasate. Binding of chymotrypsin to liposomes has been studied as a model for binding substances to liposomal surfaces. Recognizing substances, including antibodies, glycoproteins and lectins have been bound to liposomal surfaces in an attempt to confer target specificity to the liposomes. Concentrating on systemic applications and in vivo studies, these previous efforts discuss methods of binding recognizing substances with liposomes and the effectiveness of such modified liposomes. Although the bonding of these recognizing substances to liposomes occurred, the resulting modified liposomes did not perform as hoped, particularly during in vivo studies. Other difficulties are presented when utilizing these recognizing substances. For example, antibodies can be patient specific and, therefore, add cost to the drug therapy.
In addition to the problems outlined above, the prior art has failed to disclose an efficient and effective method of making bioadhesive liposomes useful for scaling-up to an industrial level. In "Preparation of EGF Labeled Liposomes and Their Uptake by Hepatocytes," Ishii et al., Biochemical and Biophysical Research Communications, Vol. 160, pp. 732-36, 1989 ("Ishii et al."), the authors describe uptake of EGF-bearing liposomes by liver cells in suspension. In the preparation of their liposomes, Ishii et al., disclose a procedure involving at least four different steps, each individually involving at least two more sub-steps. These steps include further purification by column chromatography, which can be difficult to scale-up to an industrial level. Furthermore, not only is this process cumbersome, but each additional step contributes to a loss of material or possible inactivation of the EGF. It has been reported that the biological activity of EGF is dependent upon the conservation of the native conformation of EGF, to which the disulfide bonds are critical. In binding EGF to liposomes, Ishii et al. exploited the existence of the disulfide bonds. Specifically, EGF was bound to the liposomal surface by the disulfide bridge linkage using a heterobifunctional crosslinking reagent, N-hydroxysuccinimidyl-3-(2-pyridyldithio) propionate. The complex chemistry of this process results in byproducts whose effect on drug delivery and toxicity are unknown, possibly resulting in inactivation of the EGF. Further, the complex process described by Ishii et al. would be virtually impossible to accomplish in an aseptic environment, as required in a liposome process.
Prior to the development of the present invention, a need existed for a liposome having targeting and retention abilities to a target organ or tissue. Specifically, there remains a need for the development of a "bioadhesive" liposome comprising a liposome having an effective recognizing substance attached thereto. Prior to the present invention, a need also existed for an efficient method for binding recognizing substances to a liposome thereby producing a bioadhesive liposome, using fewer steps than those described in the prior art.